The present invention relates to an infrared Fourier transformation spectrometer comprising a non-linear analog-to-digital converter device having at least one amplifier and one sample and hold circuit connected downstream thereof, as well as an analog-to-digital converter following the latter, wherein an input signal to be converted is to be supplied to one input of the amplifier and the gain is a function of the magnitude of the input signal, and wherein the output signal of the analog-to-digital converter is evaluated giving regard to the respective gain.
Among the known infrared Fourier transformation spectrometers, there is for example one offered by Bruker Analytische Me.beta.technik GmbH, D-7512 Rheinstetten, Federal Republic of Germany, as model IFS 88. An infrared interferometer incorporated in the spectrometer supplies the described input signal, namely an interferogram. The digital values supplied by the converter device are processed by Fourier transformation by an arithmetic unit, which is part of the spectrometer, in order to determine the frequency fractions of the interferogram.
Converter devices of the before-mentioned type have been known in the form of so-called floating-point analog-to-digital converters. In this case, the input signal is amplified by a variable-factor amplifier in such a way as to drive a subsequent fixed-point analog-to-digital converter module to the maximum. One achieves in this way higher resolution, by the amplification factor, and consequently a corresponding improvement of the volume range. The input signal is sampled in a sample and hold circuit. Thereafter, the optimum amplification factor is determined with the aid of comparators, and the amplifier is adjusted correspondingly. Thereafter, once the amplifier has assumed its steady-state condition, conversion can proceed. Given the fact that the sample and hold circuit has only limited resolution and holding capacity (pedestal error, drop rate), errors will arise which are then amplified as well. Consequently, high quality is needed for this sampling element. In addition, there may be cases where a different gain is adjusted for each sampling operation, so that the amplifier must assume its steady state very rapidly. The amplifier, therefore, must have a very important band width, and the amplification factors must be adjusted very precisely. Any balancing error will result in deteriorated resolution. And the other demands placed on the amplifier make it even more difficult to achieve high resolution.
DE-23 31 890 B2 shows a Fourier spectrometer comprising a single sample and hold circuit and a single analog-to-digital converter. A first signal path supplies signals characteristic of the position of the mirror, which are supplied to a sample and hold circuit as control signals. The signal to be sampled and to be converted is supplied via a different signal path.
EP-0 104 333 A2 describes an infrared Fourier transformation spectrometer which is capable of converting two different analog signals to digital values and of storing them practically simultaneously. This is achieved by operating analog-to-digital converters alternately.
From DE-39 00 247 A1 it results that it has been known before in connection with a Fourier spectrometer to have an analog-to-digital conversion preceded by an amplification step.